An inkjet recording apparatus is used as an image recording apparatus or an image forming apparatus such as a printer, a facsimile machine, a copy machine, etc. An inkjet recording apparatus is equipped with an inkjet head as a droplet-discharging head. Generally, such an inkjet head comprises: a single or a plurality of nozzles for discharging droplets of ink; a discharge chamber connecting with the nozzles; and pressure generating means for generating a pressure to pressurize the ink in the discharge chamber. The discharge chamber may be referred to as a pressurizing chamber, an ink chamber, a liquid chamber, a pressurizing liquid chamber, a pressure chamber or an ink passage. Droplets of ink are discharged from the nozzles by pressurizing the ink in the discharge chamber using a pressure generated by the pressure generating means.
Generally, a piezoelectric type, a thermal type and an electrostatic type are used for the inkjet head as a droplet discharge head. The piezoelectric inkjet head discharges droplets of ink by deforming a vibration plate (a diaphragm) that forms a wall of the discharge chamber by using an electromechanical transducer such as a piezoelectric element as the pressure generating means. The thermal inkjet head discharges droplets of ink by film boiling using an electrothermal transducer such as a heat-generating resistor provided in the discharge chamber. The electrostatic inkjet head discharges droplets of ink by deforming a vibration plate that forms a wall of the discharge chamber by an electrostatic force.
In recent years, the thermal type and the electrostatic type, which do not use parts containing lead, have attracted attention from the viewpoint of environmental issues. Especially, several electrostatic inkjet heads have been suggested from the viewpoint of low power consumption in addition to the lead-free feature.
Japanese Laid-Open Patent Application No. 6-71882 discloses an electrostatic inkjet head provided with a pair of electrodes with an air gap formed therebetween. One of the two electrodes serves as a vibration plate, and an ink chamber to be filled with ink is formed on a side of the vibration plate opposite to the electrode facing the vibration plate. An electrostatic attraction force is generated between the pair of electrodes by applying a voltage across the electrodes (between the vibration plate and electrode), which results in deformation of the electrode (vibration plate). The vibration plate returns to the original position due to an elastic force when the voltage is canceled, and a droplet of ink is discharged due to the return force of the vibration plate.
Additionally, Japanese Laid-Open Patent Application No. 2001-18383 and WO99/34979 disclose a structure of an inkjet head in which a small air gap is formed between the vibration plate and the electrode by etching a sacrifice layer, and a liquid chamber substrate is joined thereon.
Further, Japanese Laid-Open Patent Application No. 11-314363 discloses an inkjet head which can be driven at a low voltage by forming a vibration plate of a cantilever beam or a straddle mounted beam with a gap into which ink can flow, and filling a high dielectric-constant ink in the gap.
Additionally, Japanese Laid-Open Patent Application No. 9-193375 discloses an inkjet head having a vibration plate and an electrode that are positioned nonparallel to each other.
Further, Japanese Laid-Open Patent Application No. 2001-277565 discloses an inkjet head, which attempts a low-voltage drive by varying a thickness of a dielectric insulating layer formed on the electrode so as to generate a nonparallel electric field.
In the electrostatic inkjet head containing the electrostatic actuator equipped with the vibration plate and the electrode facing the vibration plate, it is necessary to make the air gap between the electrodes very small so as to achieve a low-voltage drive.
However, in the head disclosed in the above-mentioned Japanese Laid-Open Patent Application No. 6-71882, since the air gap is formed by formation of a cavity by etching and bonding a vibration plate substrate by anode junction, it is very difficult to accurately form such a small air gap with little variation, which causes a problem that the yield rate is low.
Thus, in the head disclosed in the above-mentioned Japanese Laid-Open Patent Application No. 2001-18383, although the air gap is formed with sufficient accuracy in accordance with a gap-forming method using etching of the sacrifice layer, there is a problem in that a reliability of the vibration plate is low since etching holes for etching the sacrifice layer are formed in the vibration plate. Additionally, since the approach of sealing the etching holes by an insulating layer after etching the sacrifice layer is used, the insulating layer for sealing the etching holes must be thick. Thus, there is a problem in that the rigidity of the vibration plate increases and a drive voltage increases, which causes a fluctuation in the rigidity of the vibration plate. Further, there is unevenness in the surface of the actuator substrate due to the formation of the air gap, and high alignment accuracy is required when joining a liquid chamber substrate. Moreover, since the junction area is small, it tends to cause a work mistake such as destruction due to a contact at the time of joining etc., and there is also a problem that a reliability is decreased and the yield rate is decreased.
Moreover, in the head disclosed in the above-mentioned Japanese Laid-Open Patent Application, No. 11-314363, although the air gap is formed by etching the sacrifice layer, the vibration plate has a structure of a cantilever beam or a straddle mounted beam and the air gap is communicated with the liquid chamber. In this case, since there is no need of forming the etching holes for etching the sacrifice layer and ink is allowed to enter the air gap, it is possible to achieve a low-voltage drive by using a high dielectric-constant ink which reduces an effective air gap. However, a problem tends to occur that an ink component is subjected to condensation since a voltage is applied to the ink in the gap, and there is a problem in that a high-speed drive cannot be performed due to the conductance of the ink in the gap.
Moreover, the above-mentioned Japanese Laid-Open Patent Application No. 9-193375 and Japanese Laid-Open Patent Application No. 2001-277505 do not disclose any method of forming a nonparallel air gap or any specific method for varying the thickness of the dielectric insulating layer, and, thus, a problem that it is very difficult to form a small air gap with little variation is not solved.
In the electrostatic inkjet head, the dimensional accuracy of a distance between the vibration plate and the electrode greatly affects the performance of the electrostatic inkjet head. Especially, in the case of an inkjet head, if the variation in the characteristic of each actuator is large, accuracy in printing and reproducibility of image quality goes down remarkably. Moreover, in order to attain a low-voltage operation, the size of the air gap must be 0.2 μm to 2.0 μm, which requires higher dimensional accuracy.
Japanese Laid-Open Patent Application No. 2001-18383 and WO99/34979 disclose a head constituted by forming a small air gap between the vibration plate and the electrode by applying a sacrifice layer process (etching the sacrifice layer) and joining a flow passage substrate thereon. According to this approach, the size of the air gap is determined by variation in a process of forming the sacrifice layer, and, thus, variation in the size can be suppressed, thereby obtaining an actuator or a head having high accuracy and high reliability.
Moreover, when the air gap is formed using the sacrifice layer process as mentioned above, it is necessary to seal the through holes for removing the sacrifice layer (sacrifice layer removal holes). Thus, WO99/34979 disclose that the sacrifice layer removal holes are closed by a Ni film or SiO2 film formed by a PVD or CVD method after the sacrifice layer is removed. However, if the sacrifice layer removal holes are sealed by such a film deposition method, the components of the film may enter the air gap. Additionally, the sacrifice layer removal holes also serve to maintain a strength of the partition wall, and they cannot be made small. Therefore, the sacrifice layer removal holes being sealed by the film deposition using a PVD or CVD method may influence the operation characteristic and reliability of the actuator and it cannot deal with densification.
Moreover, in the head disclosed in Japanese Laid-Open Patent Application No. 2001-18383, there is formed a step in the partition parts and the vibration plate, which requires high accuracy in joining the flow passage substrate. Moreover, since the thin vibration plate is floated on the surrounding parts after the sacrifice layer is removed, the vibration plate may be damaged in the subsequent process and it is difficult to manufacture the actuator with a sufficient yield rate.
Additionally, although the sacrifice layer removal holes are sealed by a film formed by a film deposition method using a vacuum device, the use of the vacuum device may cause a problem. If the film deposition is performed by the vacuum device, the film deposition process is performed in a vacuum environment and the air gap between the vibration plate and the electrode is sealed in vacuum. Therefore, there is a problem in that the vibration plate may be bent due to a negative pressure inside the air gap when the actuator is exposed to an atmosphere. Additionally, if there is variation in the bent of the vibration plate, there may occur variation in the displacement of the vibration plate. In addition, since the vacuum seal cannot provide a damping effect of a gas sealed in the air gap, variation in amplitude of vibration with respect to variation in the thickness of the vibration plate becomes large.
In order to solve such a problem, it is necessary to provide a structure or a process for opening the air to the atmosphere, which causes an increase in the cost and deterioration in the yield rate. Thus, if the conventional sacrifice layer process is used, it is difficult to obtain an electrostatic actuator having high-accuracy and reliability at a low cost.
In the meantime, in an inkjet recording apparatus, in order to achieve high-definition recording of a color image at high speed, high-density processing using a micro-machining technology is used to obtain a high-quality image. Thus, materials of parts constituting the head are shifted from metal or plastic to silicon, glass or ceramics. Especially, silicon is used as a material, which is suitable for the micro-processing.
Moreover, in respect of colorization, developments of ink and recording media are a main streams, and developments have been progressed with respect to components and compositions of ink so as to optimize absorbability, coloring characteristic and color-mixture prevention characteristic or improve a long-term storage of printed media and storage stability of ink itself.
In such a case, depending on a combination of ink and a material of component parts of the head, the component parts may be dissolved in the ink. Especially, if a flow passage formation member is formed of silicon, silicon is eluted in ink and is deposited on a nozzle part, which causes degradation of image quality due to nozzle clogging or deterioration of coloring of ink. Moreover, in the head using a vibration plate formed of a thin silicon film, if the silicon forming the vibration plate is eluted in ink, the vibration characteristic may be changed or the vibration plate cannot vibrates.
If the material of the component parts is changed to solve the problem, it is difficult to realize high-density processing or processing accuracy may be deteriorated in many cases. Moreover, the change in the material requires a large change in the fabrication process or assembly process, which results in decrease of nozzle density and consequently causing degradation of the print quality.
On the other hand, if the problem is solved by adjustment of components of ink, a high-quality image may be deteriorated since the components and composition of ink are originally adjusted so that permeability and coloring characteristics with respect to recording medium are optimized so as to improve the printing quality and storage stability is improved.
Thus, in the conventional inkjet head, a thin film having an ink resistance is formed on a surface of a flow passage forming member that is brought into contact with ink. For example, forming titanium, titanium compound, or aluminum oxide on the surface which contacts with ink is disclosed in WO98/42513. Forming an oxide film on the surface which contact with ink is disclosed in Japanese Laid-Open Patent Application No. 5-229118. Forming a thin film such as oxide, nitride or metal having an ink resistance on a surface of a silicon oxide film is disclosed in Japanese Laid-Open Patent Application No. 10-291322. Forming an organic resin film on a surface of the ink chamber formed of a piezoelectric material is disclosed in Japanese Laid-Open Patent Application No. 2000-246895.
In the above-mentioned head, an organic resin film such as paraxylene may be formed as a corrosion resistant film on sidewalls of an ink chamber having a complex three-dimensional configuration and the vibration plate. Since the organic resin film such as paraxylene is formed by the vacuum vapor deposition method, the covering characteristic of the film is not good due to its nature of deposition, and a large unevenness arises in the distribution of film thickness inside the liquid chamber or on the vibration plate.
When an area where the film thickness is small contacts with ink for a long time, there is a large problem arises in the long-time reliability since the corrosion resistant film is dissolved and finally the base material is subjected to corrosion. Moreover, a large bend is generated due to a distribution of internal stresses caused by variation of film thickness of the organic resin film on the vibration plate, which causes a large variation in the ink injection characteristic.
Moreover, in the head in which a metallic ink resistant film is formed on the vibration plate by a sputtering method or a vapor deposition method, the covering characteristic of the corrosion resistant film is poor similar to the above-mentioned organic resin film. Depending on the location, an area in which the corrosion resistant film is formed with a very small thickness, and when ink contacts such an area for long time, the corrosion resistant film is dissolved and finally the base material is subjected to corrosion. Therefore, a long-time reliability cannot be obtained, and further a large bent is generated in the vibration plate due to fluctuation in the thickness of the metallic ink-resistant film, which causes variation in the ink injection characteristic.
Especially, this problem is serious in the electrostatic head rather than the piezoelectric head since the distance between the vibration plate and the electrode varies due to the vibration plate being bent and the drive voltage differing from the design value.
Further, in the head in which the above-mentioned corrosion resistant film is formed, the reliability of operation is low such that the vibration plate contacts the electrode due to an influence of an external environment such as humidity since the air gap between the vibration plate and the electrode is not sealed.
Moreover, in the head in which the air gap between the vibration plate and the electrode is sealed so as not to receive an influence from an external environment, there is a restriction of pH value of ink that is usable since the corrosion resistant film is not formed on the vibration plate, and, thus, matching with ink must be maintained and a cost is increased.